Effect of molecular structure on polyethylene melt rheology. I. Low‐shear behavior

Abstract

The melt rheological properties of both linear and branched polyethylene were investigated by use of narrow molecular weight distribution fractions and experimentally polymerized samples. Studies carried out in steady shear and in oscillatory shear yielded information concerning both the melt viscosity and the melt elasticity as a function of molecular structure, where the latter was characterized by various solution property techniques. The 3.4–3.5 power dependence of the low shear limiting viscosity on molecular weight was confirmed for linear polyethylene. The effect of long‐chain branching on rheological properties was defined both at constant molecular weight and at constant molecular weight distribution and coupled with variation of molecular weight.

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

“…Another common measure of LCB arises when the intrinsic viscosity of the polymer is compared with that of a linear chain of the same M w [7,40]:…”

“…Fig. 2 shows the results obtained for fractions of linear and branched PE by these authors [7]. [7] When the LCB content is very low (less than 1 LCB /10 4 C), its effect on the radius of gyration goes unnoticed and values of g ≈ 1 are obtained [42].…”

“…Using a value of ε = 0.5, Mendelson et al [7] demonstrated that comparisons made at a constant effective molecular volume, given by the product gM w , yielded values of η o for polymer melts with long branches that were exponentially related to molecular size [43] and always higher than those recorded for linear polymers. A similar conclusion was reached for the onset of pseudoplastic behaviour.…”

“…However, despite extensive work on the effects of LCB on the viscoelastic properties of these types of material in the melt in the 1960s, there is no unified picture of their dependence on molecular variables. The following general properties were established for a moderate to high degree of LCB (>> 1 LCB/10 4 carbon atoms): a) lower Newtonian or zero-shear viscosity η o and a higher critical shear rate o γ& for the onset of shear thinning behaviour than linear polymers of the same weightaverage molecular weight, M w [3][4][5][6][7][8][9][10][11][12][13][14]; b) less intense pseudoplastic behaviour [11,[15][16][17]; c) increased activation energy of flow, E a [18][19][20][21][22][23][24][25][26][27][28][29]; and d) enhanced melt elasticity expressed in terms of first normal stress difference N 1 , steady-state compliance J e o and extrudate swell d j /D [4,5,12,13,16,17,[30][31][32].…”

Rheology of Star-Branched Polyisobutylene

“…[10][11][12][13][14][15][16][17][18] This arm retraction is believed 19 to be the cause of two other phenomena associated with branched polymers: more temperature-dependent rheological properties and thermorheological complexity in the terminal zone. [20][21][22][23][24][25][26][27] The transient, compact structure would alter the distribution of rotational states, in turn giving rise to a thermal barrier to terminal relaxation, which is absent for linear polymers. If the gauche rotamers, which predominate in a more compact configuration, are higher in energy than the trans conformers, the model of Graessley 19,20 predicts the existence of an energy barrier, E A , whose magnitude is proportional to the molecular weight, M a , of the branch where M e is the entanglement molecular weight.…”